Recently, in a field using a steel material such as construction industrial machinery, a tank, penstock and a line pipe, a steel material to be used is oriented to be increased in strength, and use environment of a steel material becomes progressively harsher with increase in size of structures as background.
However, it is known that such increase in strength of a steel material and increase in harshness of use environment typically increase hydrogen embrittlement sensitivity of the steel material, and for example, in a field of high-strength bolt, a high strength steel material is restrictively used, for example, JIS B 1186 describes that F11T class bolt (with tensile strength of 1100 to 1300 N/mm2) is preferably not used.
Therefore, documents described below, that is, patent document 1, patent document 2, patent document 3, patent document 4, and patent document 5 have proposed a method of manufacturing a steel sheet having an excellent hydrogen embrittlement resistance property using various techniques such as optimization of a composition, reinforcement of grain boundaries, refinement of crystal grains, use of hydrogen trap sites, structural morphology control, and fine dispersion of carbides.    [Patent document 1] JP-A-3-243745    [Patent document 2] JP-A-2003-73737    [Patent document 3] JP-A-2003-239041    [Patent document 4] JP-A-2003-253376    [Patent document 5] JP-A-2003-321743